Japan Coast Guard personnel prepare for search and rescue operations, to search for the missing Malaysia Airlines MH370 plane over the waters of the South China Sea on March 15, 2014. Investigators believe someone aboard a missing Malaysian airliner deliberately shut off its communications and tracking systems, turned the plane around and flew for nearly seven hours after it vanished, Prime Minister Najib Razak said on Saturday. Credit: Reuters

Malaysian investigators are trawling through the backgrounds of the pilots, crew and ground staff who worked on a missing jetliner for clues as to why someone on board flew it perhaps thousands of miles off course, the country's police chief said.

Background checks of passengers on Malaysian Airlines Flight MH370 have drawn a blank, but not every country whose nationals were on board has responded to requests for information, police chief Khalid Abu Bakar told a news conference on Sunday.

No trace of the Boeing 777-200ER has been found since it vanished on March 8 with 239 people on board, but investigators believe it was diverted by someone who knew how to switch off its communications and tracking systems.

Malaysia briefed envoys from nearly two dozen nations and appealed for international help in the search for the plane along two arcs stretching from the shores of Caspian Sea to the far south of the Indian Ocean.

"The search area has been significantly expanded," said Acting Transport Minister Hishammuddin Hussein. "From focusing mainly on shallow seas, we are now looking at large tracts of land, crossing 11 countries, as well as deep and remote oceans."

The plane's disappearance has baffled investigators and aviation experts. It vanished from civilian air traffic control screens off Malaysia's east coast less than an hour after taking off from Kuala Lumpur en route to Beijing.

Malaysian authorities believe that as the plane crossed the country's northeast coast and flew across the Gulf of Thailand, someone on board shut off its communications systems and turned sharply to the west.

Electronic signals it continued to exchange periodically with satellites suggest it could have continued flying for nearly seven hours after flying out of range of Malaysian military radar off the country's northwest coast, heading towards India.

Malaysian officials briefed ambassadors from 22 countries on the progress of the investigation and appealed for international cooperation, diplomats said on Sunday.

Although countries have been coordinating individually, the broad formal request marked a new diplomatic phase in a search operation thought increasingly likely to rely on the sharing of sensitive material such as military radar data.

"The meeting was for us to know exactly what is happening and what sort of help they need. It is more for them to tell us, 'please put in all your resources'," T.S. Tirumurti, India's high commissioner to Malaysia, told Reuters.

Malaysian Prime Minister Najib Razak also telephoned his Indian counterpart, Manmohan Singh, to ask for Indian help corroborating possible paths taken by the jet, an Indian Foreign Ministry spokesman said.

PILOTS' HOMES SEARCHED

On Saturday, police special branch officers searched the homes of the captain, 53-year-old Zaharie Ahmad Shah, and first officer, 27-year-old Fariq Abdul Hamid, in middle-class suburbs of Kuala Lumpur close to the international airport.

An experienced pilot, Zaharie has been described by current and former co-workers as a flying enthusiast who spent his off days operating a life-sized flight simulator he had set up at home.

Police chief Khalid said investigators had taken the flight simulator for examination by experts.

Earlier, a senior police official said the flight simulator programs were closely examined, adding they appeared to be normal ones that allow players to practice flying and landing in different conditions.

Police sources said they were looking at the personal, political and religious backgrounds of both pilots and the other crew members. Khalid said ground support staff who might have worked on the plane were also being investigated.

A second senior police official told Reuters investigators had found no links between Zaharie, a father of three grown-up children and a grandfather, and any militant group.

Postings on his Facebook page suggest the pilot was a politically active opponent of the coalition that has ruled Malaysia for the 57 years since independence.

A day before the plane vanished, opposition leader Anwar Ibrahim was convicted of sodomy and sentenced to five years in prison, in a ruling his supporters and international human rights groups say was politically influenced.

Asked if Zaharie's background as an opposition supporter was being examined, the first senior police officer would say only: "We need to cover all our bases."

Malaysia Airlines has said it did not believe Zaharie would have sabotaged the plane, and colleagues were incredulous.

"Please, let them find the aircraft first. Zaharie is not suicidal, not a political fanatic as some foreign media are saying," a Malaysia Airlines pilot who is close to Zaharie told Reuters. "Is it wrong for anyone to have an opinion about politics?"

Co-pilot Fariq was religious and serious about his career, family and friends said.

The two pilots had not made any request to fly together.

EXPERTS DOUBT MILITANT GROUPS INVOLVED

With no clear motive established as to why someone diverted the plane, Khalid said all possibilities - hijack, sabotage, or personal or psychological problems of someone on board - were being investigated. Transport Minister Hishammuddin said authorities had not received any ransom or other demand.

Southeast Asia's homegrown Islamist militant groups, such as Jemaah Islamiah (JI), which carried out the Bali bombings in 2002, have been quiet in recent years after security forces either arrested or shot dead numerous members.

Experts said they doubted the remaining militants had the skills or capabilities to carry out a complex hijacking.

"JI has not been involved with violence in the region since 2007," said Sidney Jones, director of the Jakarta-based Institute for Policy Analysis of Conflict.

"The other groups that are active in Indonesia, in trying to make terrorist plots, are all not very competent. I would be extremely surprised if any group from Indonesia, the Philippines or Malaysia itself would be directly involved."

Analysis of satellite data showed the last signal from the missing plane was at 8:11 a.m., almost seven hours after it turned back over the Gulf of Thailand and re-crossed the Malay peninsula.

Experts could determine only that the plane could have been anywhere along either of two arcs: one stretching from northern Thailand to the border of Kazakhstan and Turkmenistan, or a southern arc heading from Indonesia to the vast southern Indian Ocean.

Hishammuddin said Malaysia had requested further satellite data from several countries, including the United States, China and France, to help with the search.

A source familiar with official U.S. assessments said it was thought most likely the plane had headed south into the Indian Ocean, where it would presumably have run out of fuel and crashed. Air space to the north is much busier, and the plane would likely have been detected.

A Republican politician involved in intelligence expressed frustration on Sunday that U.S. experts from agencies such as the FBI were not more involved in the investigation.

"The fact is the FBI was not asked in. And you know these pilots they should have been - pilot and co-pilot - should have been the focus from the start," U.S. Representative Peter King, chairman of the House Counterterrorism and Intelligence Subcommittee, told the ABC program "This Week."

Countries contacted by Malaysia to assist in the search range from the former Soviet central Asian republics in the north to Australia in the south, along with France, which administers a scattering of islands deep in the southern Indian Ocean, uninhabited except for a handful of researchers.

France, which had four of its citizens on the plane, said on Sunday it had agreed to send three special aviation investigators to Kuala Lumpur.

The Indian Ocean is one of the most remote places in the world and also one of the deepest, posing enormous challenges for efforts to find any wreckage or the flight voice and data recorders that would be key to solving the puzzle.